Problem: 7 people can paint 4 walls in 42 minutes. How many minutes will it take for 9 people to paint 5 walls? Round to the nearest minute.
We know the following about the number of walls $w$ painted by $p$ people in $t$ minutes at a constant rate $r$ $w = r \cdot t \cdot p$ $\begin{align*}w &= 4\text{ walls}\\ p &= 7\text{ people}\\ t &= 42\text{ minutes}\end{align*}$ Substituting known values and solving for $r$ $r = \dfrac{w}{t \cdot p}= \dfrac{4}{42 \cdot 7} = \dfrac{2}{147}\text{ walls painted per minute per person}$ We can now calculate the amount of time to paint 5 walls with 9 people. $t = \dfrac{w}{r \cdot p} = \dfrac{5}{\dfrac{2}{147} \cdot 9} = \dfrac{5}{\dfrac{6}{49}} = \dfrac{245}{6}\text{ minutes}$ $= 40 \dfrac{5}{6}\text{ minutes}$ Round to the nearest minute: $t = 41\text{ minutes}$